A new threat linked to climate change looms over the green turtle, a species declared in danger of extinction on the Red List of Threatened Species of the International Union for Conservation of Nature (IUCN). Pollution can cause an excessive birth of females, according to an article published this Monday in the journal Frontiers in Marine Science.
The green turtle (Chelonia mydas) is at risk of extinction due to poaching, collisions with boats, destruction of its habitat, and accidental capture in fishing gear.
But another threat associated with climate change is more insidious because sea turtles have temperature-dependent sex determination, meaning more and more embryos become females as temperatures continue to rise. In the northern part of the Great Barrier Reef off Australia, hundreds of females are already born for every male.
Now, researchers have shown that the risk of extinction from a lack of male green turtles can be exacerbated by pollution.
“Pollutants from human activities may also influence the sex ratio of developing green sea turtles, increasing the already existing bias towards females,” according to Arthur Barraza of the Australian Rivers Institute at Griffith University and first author of the study.
Barraza and his colleagues studied the effects of pollution on the development of green turtles on Heron Island, a small coral sand cay in the south of the Great Barrier Reef. This island, where between 200 and 1,800 females nest each year, is a long-term monitoring site for this species. There, the sex ratio is more balanced than near the equator, with two or three females for every male.
During the study, researchers did everything possible to minimize animal suffering and maximize the amount of data obtained from each deceased turtle hatchling. The work was approved by the animal ethics committee of the University of Queensland and Queensland Parks and Wildlife Services.
The authors collected 17 entire clutches within two hours of laying the eggs and reburied them near automated temperature probes. The team recorded the temperature inside the nest and on the beach surface every hour.
When the hatchlings emerged, they were sacrificed and their sex was determined by dissecting and examining the sexual organs. Their livers were also removed and the contaminants it contained were measured with inductively coupled plasma mass spectrometry, as well as bioassays on cultured sea turtle cells.
The authors focused on 18 carbon metals, such as chromium, antimony, and barium, as well as organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs). All of them function as ‘xenoestrogens’, that is, molecules that bind to the receptors of female sex hormones.
“The accumulation of these contaminants by a female turtle occurs in the place where it searches for food. As the eggs develop inside it, they absorb the contaminants that have accumulated. Then they are sequestered in the liver of the embryos, where they can remain for years after hatching,” says Barraza.
The final sex ratio varied from 100% males to 100% females among clutches, although most nests produced primarily female offspring. The higher the average amount of antimony and cadmium and heavy metals in the liver of the young, the greater the tendency towards females within the nest.
The authors conclude that these pollutants mimic the function of the hormone estrogen and tend to redirect developmental pathways toward women.
“As the sex ratio approaches 100% female, it will be increasingly difficult for adult female turtles to find a mate. This becomes especially important, as climate change will continue to make nesting beaches warmer and more favored by females,” says Barraza.
Jason van de Merwe, also from the Australian Rivers Institute, highlights that, “given that most heavy metals come from human activities such as mining, runoff and pollution from general waste in urban centres, the best way to Moving forward is using long-term strategies based on science” to reduce the entry of pollutants into the oceans.
